Object not visible in uv editor
The problem is that you are working in Local viewport.



You can swith from Local viewport to standard viewport and viceversa by pressing "/" key (on the numpad, or Shift+7). This will affect also the UV/Image editor (you can see the UVs only for the objects you see in local viewport).

The local viewport mode is useful because you can see only the selected objects, but you don't have to hide the other objects in the scene. Just select your object(s), press "/" and you see only them. Press "/" again and you will see all objects in the scene.